Structures that are applied to a substrate by means of lithography technology during the production of integrated semiconductor circuits have to be made smaller and smaller. A technologically limiting factor in this case is the wavelength of the light used during the lithography. On account of diffraction effects, the resolution of a conventional imaging system is limited and structures having dimensions below the reciprocal value of said resolution, the so-called critical structures, are blurred or imaged unsharply. This can lead to impairments of the circuit.
These difficulties can be overcome by utilizing the destructive interference effect of two closely adjacent and coherent light beams with phases shifted approximately through 180°. Use is made of so-called alternating phase masks, in which each critical structure is alternately provided with two phase-shifting regions that are assigned a different phase shifted through about 180°. A destructive interference effect thereby arises on the substrate to be patterned, for example a semiconductor circuit, said destructive interference effect enabling the formation of the critical structures.
In this case, a distinction is made between dark field technology, in which the transparent structures correspond to the circuit elements (e.g., the interconnects) and nontransparent mask fields are formed by zones covered with chromium, for example, and bright field technology, in which the nontransparent structures correspond to the circuit elements (e.g., the interconnects) and the transparent mask fields form the free zones.
Since modern circuits such as VLSI and ULSI circuits are becoming ever more complex and the critical structures assume very complicated geometrical forms, the phase allocation with only two different phase-shifting regions is complicated. Conflict regions (also called phase conflicts) may occur at critical structures, and, during the lithography operation, cause imaging errors on the substrate to be patterned. In the case of phase conflicts, a mask region is not assigned the required phase difference. This is the case if a critical structure has incorrectly been allocated the same phase on both sides, which leads to irreparable damage to the semiconductor circuit during the exposure operation. On the other hand, however, conflict regions may also occur if a destructive interference effect occurs at an undesirable location of the semiconductor circuit on account of the interaction of the phase-shifting elements.
The phase allocation for the different phase-shifting elements represents a mathematical-combinatorial problem, which is not generally solvable. Since the phase allocation can, in principle, lead to different results, the phase allocation has to be finally performed on the finished circuit layout in an automated program.
Published German Patent Application 100 57 438 discloses a method for checking the imagability of an alternating bright field phase mask by examining the phase mask for conflict centers of phase conflicts and localizing them. Published German Patent Application 100 57 437 discloses an analogous method for dark field technology. A conflict center is an inner contour of a contiguous region comprising critical structures and phase-shifting regions, adjoined by an odd number of critical structures. At such a conflict center, a phase-conflict-free phase allocation for the phase-shifting regions is not possible combinatorially, that is to say a phase conflict cannot be avoided. U.S. Patent Application Publication 2003/0140331, which is incorporated herein by reference, claims priority to the two above-mentioned German patent applications.
These two documents explain that phase conflicts that have occurred can be circumvented in two different ways. Firstly, the circuit layout can be altered slightly, for example by shifting the interconnect structures, thereby eliminating the phase conflicts. On the basis of this altered circuit layout, it is then possible to carry out a renewed phase allocation for the creation of the phase mask. Secondly, phase-shifting regions can be allocated two different phases. However, the consequence of this is that, at the boundary line between the two different phase zones, a dark line occurs during exposure, which would lead to an interruption of the circuit. These dark lines must thereupon be corrected.
The problem with these methods is that the circuit layout is to be altered only to a very limited degree, since the functioning of the circuit must not be altered. On the other hand, correcting the phase conflicts is time-consuming, since all of the phase conflicts have to be treated individually.